The present invention relates to a sensing system comprising a set of optical responding sensors (a set consisting of at least one sensor) for detecting the presence of at least one target gas and, more particularly, to an apparatus incorporating two sensors in the sensing system, and a method for determining the concentration of at least one target gas by monitoring the optical response of the sensor within a given period of time, for use as monitoring devices in a wide variety of applications, such as home safety, occupational health, industrial, commercial, recreational and medical, and specifically to meet the British Standard Institute carbon monoxide standard.